This article systematically explores the application of insights from behavioral economics to international legal issues. Economic analysis has in recent years made significant inroads into the study of international law, but most of this literature relies upon assumptions of perfect rationality of states and decision-makers. This approach is inadequate, both in its insufficient empirical grounding and in its question-begging tendency towards often unsophisticated and outdated forms of ‘Realist’ international relations theory. A behavioral approach would augment legal research by providing new hypotheses to address puzzles in international law while at the same time introducing empirically grounded concepts of real, observed bounded’ rationality, which diverge from the assumed, perfect rationality of traditional law and economics. The article addresses some possible methodological objections to the application of behavioral analysis to international law, namely: the focus of behavioral analysis on the individual; the empirical foundations of behavioral economics; and behavioral analysis’ relative lack of parsimony. It then offers indicative behavioral research frameworks for three outstanding puzzles in international law: (a) the relative inefficiency of the development of international law; (b) collegiality and dissent in international tribunals; and (c) target selection in armed conflict. Behavioral research of international law can serve as a viable and enriching alternative and complement to economic analysis and other theoretical approaches to international legal research, so long as it is pursued with academic and empirical rigor as well as intellectual humility.